1. Field of the invention
The present invention refers to a circuit for current and voltage control in a switching power supply controlled by the primary side. The switching power supply comprises a transformer with a primary-sided and secondary-sided main winding, wherein electrical energy pulses are transmitted from the primary side to the secondary side. A primary-side switch switches on and off a primary current through the primary-sided main winding. Furthermore, the invention refers to a method of controlling the output voltage and the output current in a switching power supply controlled on the primary side.
2. Description of the Related Art
The operation of electrical devices usually requires a precisely defined supply voltage so that these devices operate properly, wherein this supply voltage is in many cases independent of the mains voltage provided by the electricity network. Consequently, many electric devices, such as computers or television sets, comprise a power supply unit. Battery-supplied apparatus also have power supply units to keep the internal operating voltages constant independent of the charging condition of the battery.
The function of the power supply unit is the conversion of a supply voltage, mostly the mains voltage, into a higher or lower supply voltage required for the supply of the electrical devices. The power supply voltage is applied to a primary side of the power supply unit and the electric device is connected to a secondary side.
Furthermore, a direct ohmic connection between the electricity network system and the electrical device is not allowed for safety reasons. Thus, the primary and the secondary side are separated from one another galvanically so that the high alternating voltage of the electricity network cannot reach the electric devices. This is implemented by a transformer, which also transfers electrical energy from the primary side to the secondary side.
The output power must be controlled on the secondary side to enable a safe operation of the devices. The control required for this purpose can be implemented either on the primary side or on the secondary side, wherein different advantages and disadvantages are achieved. Power supply units controlled by the secondary side often do not have a galvanic separation between input and output. Consequently, these units are used where a galvanic separation already exists, such as in battery-operated apparatus.
The various embodiments disclosed herein refer to a switching power supply controlled on the primary side.
It is known that switching power supplies do internally not operate with the frequency of the alternating voltage of the electricity network, which in the European electricity network is at approx. 50 Hz, but at a higher clock frequency, which is usually above 20 kHz.
This increase of the frequency is carried out by the primary-sided switch, which may for instance be formed by at least one MOSFET. As an alternative, insulated gate bipolar transistors (IGBT) can also be used for this purpose. The required direct voltage is generated in a rectifier unit from the alternating voltage of the electricity network. Subsequently, the direct voltage is switched on an off with a predetermined frequency, whereby an alternating voltage with the respective frequency is produced.
The advantages of switching power supplies compared to conventional linearly controlled power supply units are the significantly higher frequency, by means of which the transformer is controlled internally. Since the required amount of windings of the transformer drops inversely proportional to the frequency, the copper loss can thereby be significantly reduced and the transformer required becomes significantly smaller. The power supply unit can be built with a lower weight and more compact, since the transformer used does no longer need to have a heavy iron core. Moreover, other components of the switching power supply can be built smaller which leads to a reduction in costs.
A disadvantage of switching power supplies are the audible noises that may be generated. The noise is on the one hand produced by the switching currents which have a significantly higher energy when the frequencies are higher.
On the other hand, noise is produced by the frequency-controlled control of the switching power supply. The reason for this is the fast switch on and off of the transformer, by which, if the frequency is in a frequency range audible by human beings, a humming or buzzing can be heard.
As already mentioned, the output shall be galvanically separated from the input, thus, the control of the transformer is implemented by the generated high-frequency alternating voltage. The transformer comprises at least one primary-sided and at least one secondary-sided main winding, which are magnetically coupled to one another. A switch on the primary-sided main winding is usually used to switch the current through the primary winding on and off. In this manner the electric energy is charged into the primary-sided main winding.
The required output power is output at the secondary-sided winding, wherein the energy of the primary-sided winding is transmitted after each charging to the secondary-sided winding.
Thus, energy pulses with a high clock frequency are taken from the electricity network through the primary-sided switch and are transmitted through magnetic coupling of the primary and secondary winding to the output. The actual transmission or conversion of the energy may take place at different points in time, according to which a distinction is made between blocking converter, flow converter and resonance converter.
In the following, only the case of the blocking converter shall be observed, in which during the blocking phase of the switch, i.e. if no current flows through the primary-sided main winding, the energy transmission from the primary side to the secondary side takes place.
If a direct voltage is to be generated as output voltage, as is the case in many consumers, such as household appliances, mobile telephones, PC and the like, the alternating voltage induced at the secondary winding must be converted into a direct voltage in an additional secondary-sided rectifier stage. A low pass filter smoothens the direct voltage additionally and therefore reduces waviness of the output voltage.
The control of the output power is usually implemented via a closed control loop, wherein it is the target to keep the output voltage constant under all operating conditions. A known solution for generating a control variable is the feedback by means of a primary-sided auxiliary winding, as is for instance shown in WO 2004/082119 A2. A voltage pulse is induced in the primary-sided auxiliary winding after switching off the primary-sided switch. This pulse is used to generate an auxiliary voltage, which is proportional to the output voltage. The control takes care that the voltage lies within the control range at the auxiliary winding. Since this information can only be gained during the blocking phase, i.e. during the time at which the switch is switched off, a sample holding circuit is included which holds the voltage value for pause times and flow phase.
The value of the auxiliary voltage forms the actual value and is compared to a reference voltage, which represents the target value. The difference between the actual value and the target value, i.e. the control deviation, influences the control of the primary-sided switch so that the energy transmitted can be adjusted. Mains fluctuations as well as changes of the load current are controlled by the control circuit.
Both the control of the frequency as well as the pulse width are determined by the switch control. The switch-on point in time defines the duration of the blocking phase and thus the frequency. The switch-off time determines the current flow duration and thus the pulse width that corresponds to the energy to be transmitted.
A known control circuit for such a primary side controlled switching power supply can be derived from the published international patent application WO 2004/082119 A2. The structure of this known arrangement is shown in FIG. 8 and its function shall now be explained in detail.
The most important components include a transformer, which galvanically separates the primary and the secondary side and comprises a primary-sided PW and secondary-sided main winding VS. The two main windings are magnetically coupled so that electrical energy pulses can be transmitted from the primary side to the secondary side.
The energy flow in the primary-sided main winding is controlled by a primary-sided switch T1. By switching on and off the switch T1, the primary current may be switched off by the primary-sided main winding PW. The energy stored in the primary-sided main winding depends on the amount of current through the winding at the time of switch-off. The higher the current, the higher the stored energy that is subsequently transmitted to the secondary side.
The transmission of the energy pulse takes place during the blocking phase of the switch T1, that means at a time when no current flows through the switch and the primary-sided main winding. The switching of the transistor T1 is controlled by a driver 801, which is supplied by the supply voltage VP.
Additionally, the transformer has a primary-sided auxiliary winding HW, in which after switching off the primary-sided switch T1, a voltage pulse is induced. The induced voltage pulse at the auxiliary winding is proportional to the output voltage, wherein the output voltage depends on a load applying at the secondary side. The level of the induced voltage pulse at the auxiliary winding is used as control variable and controlled such that it lies within a control range.
The actuator member for the control is formed by the transistor T1 and its switch-on and off point in time. FIG. 6 shows the course of the control signal G for the primary-sided switch T1 of the arrangement of FIG. 8. The switch-off duration tout and thus the clock frequency can be controlled by the switch-on point in time ttin. The energy pulse width tin may be adjusted by the switch-off time ttout and thus the energy to be transmitted in the primary-sided main winding.
The switch-off point in time is determined by means of a primary current comparator 802. The primary current flowing through the primary-sided main winding PW and the transistor T1 also flows into the resistor R5, wherein the current causes a voltage drop at the resistor R5. This voltage drop is compared at the primary current comparator 802 to a fixed reference voltage. If the reference voltage is exceeded, the driver 801 switches the control signal G such that the transistor T1 is no longer conductive and blocks. The switch-off point in time in the prior art is therefore not influenced by the control variable at the primary-sided auxiliary winding so that the same energy quantity per pulse is transmitted.
The switch-on time is determined by the control variable. Since this information can only be gained during the blocking phase, i.e. when the switch is switched off, a sample and holding circuit S&H is included which holds the voltage value for pause time and flow phase.
The control variable influences the determination of the switch-on point in time of the primary-sided switch T1. The control variable is compared to a reference voltage Exp.Ref exponentially rising over the time. If the exponential reference voltage has reached the control variable, the transistor T1 is switched on and a current flows through the primary-sided main winding.
In the arrangement shown in reference WO 2004/082119 A2 the output voltage changes with the load applying at the secondary side. To compensate these changes, the clock frequency is increased or decreased. This takes place only through the blocking phase, i.e. through the switch-on point in time. The switch-off time always remains equal relative to the switch-on time and thus to the current flow duration.
When the load is low, less energy is consumed at the output side. Since per pulse the same energy is transmitted onto the output side, the clock frequency of the switch control must be decreased to transmit less energy. If the load applying at the output side rises again, the clock frequency is increased respectively, to provide a larger energy quantity at the secondary side.
As already mentioned, the problem exists in the known control circuit, that the output voltage depends on the load, since the frequency is adjusted depending on the deviation of the auxiliary voltage from the target value. When the load is low, the output voltage and corresponding therewith the auxiliary voltage at the auxiliary winding rises. With an increasing load the output voltage drops and thus the auxiliary voltage at the auxiliary winding drops correspondingly.
A further problem of the cited control circuit is that in the case of a low load energy-rich pulses with low frequencies are switched, wherein audible noise can be produced. A problem connected therewith is also the relatively large waviness of the output voltage.
A further problem is the susceptance to failure of the clock frequency control. In this control the voltage taken at the auxiliary winding is compared to an exponential reference to determine the time at which the switch is switched on again so that current flows in the winding. The reference voltage rises approximately according to a function U−ref =k(1−e(−t/tau)) until it corresponds to the tapped voltage.
The output of the sample and holding device is kept constant for a short period of time during which the comparison with the exponential reference takes place.
The exponential reference voltage only slowly approximates to the auxiliary voltage kept constant especially when the switching times are longer. For a comparatively long period of time before the intersecting point of the exponential reference voltage with the output voltage kept constant, the difference between the reference and the output voltage is therefore small, which may lead to significant interferences of the determination of the time. This is shown as a curve 701 in FIG. 7.